The present invention relates to a method of reshaping a signal produced by reading data recorded on an optical disc. It is applicable in particular for reshaping synchronising pulses derived from data prerecorded on the disc and serving the purpose of rephasing the clock giving the clock cycles The invention equally relates to a device employing this method.
The recording methods are well known to those versed in the art and lie beyond the scope of the present invention. These data are most frequently recorded in the form of microreliefs along tracks having the form of a spiral or of concentric circles. To facilitate random access to a recorded data item a recording is partitioned into blocks or sectors.
At the time of recording user information, the different binary elements are recorded along a track at evenly spaced positions within each data block alloted to this user information. Synchronising means should consequently be provided to subsequently enable the retrieval of the rank assigned to each element recorded. To perform a reading operation it is necessary to generate timing signals for the purpose of retrieving the position of each binary element in the recorded sequence.
According to a first approach, in the so-called multitrack systems, each track alloted for recording user information is associated with at least one timing track along which are recorded clock signals permitting a synchronized reading of the user information. In a given embodiment, the synchronisation tracks are precut and comprise optically detectable and evenly spaced markings. During the reading operation with a multibeam readout device, a first optical beam is focussed on the user information track for reading the recorded data, and a second optical beam which is separate from the first but in a condition of constant mechanical coupling with the first, is utilised to read the synchronising data stored in the timing track. With a single beam readout device both the user information track and the timing track are scanned with the same optical beam. In this case, discrimination should be performed between the two kinds of data. By way of example, the frequency spectra respectively allocated to user and timing data should be different. In this later case precut synchronising data may be utilised while writing in the user information.
In order to increase the recording density, the use a single track has been suggested. In this case, the synchronising clock signals may be derived from reading the user data itself.
In order to simplify synchronisation, writing in the user data is conventionally done with codes described as being self-synchronising or as having a maximum of transitions, since such codes provide synchronization whatever the content of the user data stream. To store data coded in this fashion, the recording is made of microreliefs which have two reference levels, respectively allocated to transcribe the logic values "0" and "1". The synchronising signals are derived from the detection of the transitions occuring between these levels, and these transitions are utilised as timing signals for the frequency and phase synchronization of a turntable oscillator, conventionally of the kind utilising voltage control (known by the abbreviation VCO. This oscillator is electrically controlled with a PLL device receiving these transitions and the voltage supplied by the oscillator (PLL: "phase locked loop"). Futhermore, special bursts of recorded pulses are utilised to initiate clocking.
However, this kind of coding does not permit maximum recording density. It is also known, for increasing this density, to make use of codes which are not self-synchronising, for example the NRZ code (non-return to zero). This kind of code has the characteristic that it lacks a transition from one information bit to the next one when these two bits have the same logic value. The timing signals needed for synchronisation are then more difficult to derive by reading the information coded in this manner. Synchronising means operating with such codes are however known, for example, from the U.S. Pat. No. 4,566,092 which describes a method and device in which the synchronisation elements or "flags" are prerecorded at uniformly spaced intervals along the tracks of the disc. Each time a flag runs past beneath a pickup head, the circuits operating with synchronising signals resynchronise the write-read system and in particular a clock circuit providing the write-read cycles. To this end, it is appropriate to have available synchronising signals having substantially constant duration and cyclic ratio, which may be compared to those of the clock signals.
As is apparent from the foregoing, these synchronising signals originate from specific data elements prerecorded on the disc. by reason of data arrangement on the disc, two corresponding data elements respectively located on a peripheral track and on a central track of the disc, are of different length. Furthermore, the recording of these data elements occurs either by optical means or by pressing, and may give rise to distortion. Similarly, the reading of these data elements is affected by deformations caused by imperfections of the reading systems, by mamufacturing tolerances, and by other factors such as the efficiency and quality of the optical devices, the sensitivity of the cells and the power of the light source.